A passive optical network (PON) is a promising approach regarding fiber-to-the-home (FTTH), fiber-to-the-business (FTTB) and fiber-to-the-curb (FTTC) scenarios, in particular as it overcomes the economic limitations of traditional point-to-point solutions.
Several PON types have been standardized and are currently being deployed by network service providers worldwide. Conventional PONS distribute downstream traffic from the optical line terminal (OLT) to optical network units (ONUs) in a broadcast manner while the ONUs send upstream data packets multiplexed in time to the OLT. Hence, communication among the ONUs needs to be conveyed through the OLT involving electronic processing such as buffering and/or scheduling, which results in latency and degrades the throughput of the network.
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colors) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber.
WDM systems are divided into different wavelength patterns, conventional or coarse and dense WDM. WDM systems provide, e.g., up to 16 channels in the 3rd transmission window (C-band) of silica fibers of around 1550 nm. Dense WDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system may use 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. Some technologies are capable of 25 GHz spacing. Amplification options enable the extension of the usable wavelengths to the L-band, more or less doubling these numbers.
Optical access networks, e.g., coherent Ultra-Dense Wavelength Division Multiplex (UDWDM) networks, are deemed to be used as a future data access.
Upstream signals may be combined by using a multiple access protocol, e.g., time division multiple access (TDMA). The OLTs “range” the ONUs in order to provide time slot assignments for upstream communication. Hence, an available data rate is distributed among many subscribers. Therefore, each ONU needs to be capable of processing much higher than average data rates. Such an implementation of an ONU is complex and costly.
Data transmission of spectrally densely spaced wavelengths is utilized by applications such as Next Generation Optical Access (NGOA) systems allowing high data rates of, e.g., 100 Gbit/s.
However, NGOA systems provide symmetric bandwidth in upstream and downstream directions. Hence, NGOA approaches waste a significant amount of resources in case applications mainly utilize an asymmetric bandwidth profile. Such resources wasted in particular comprise: An optical spectrum, a processing capacity of a software radio in an upstream receiver, a packet processing capability in upstream direction, a downstream capacity that cannot be used by other subscribers due to the allocated upstream resources.
On the other hand, the downstream bandwidth of NGOA may be fixed per subscriber, hence bandwidth cannot be dynamically allocated to a specific subscriber for a short period of time.
However, GPON or GEPON are known as applications with an asymmetric bandwidth profile as well as a high overbooking factor. A main disadvantage relates to a ranging procedure and the dynamic bandwidth allocation in upstream direction, which results in transmission delays amounting to 20-100 ms as well as a complex implementation of an according protocol. It is a further disadvantage that the upstream transmitters of the ONUs are switched on and off due to the time-domain multiplexing, resulting in high dynamic requirements at the upstream receiver. As GPON and GEPON do not use heterodyne receivers, the power budget (length, splitting factor) is also limited.
The international application WO 2011/110223 describes a system where the upstream wavelength is locked to the downstream wavelength, and in which an oscillator and a mixer form an additional stage in the receive path which shifts the intermediate frequency.
The problem to be solved is to overcome the disadvantages stated above and in particular to provide an efficient approach of an asymmetric NGOA system, which is also compatible with GPON systems.